This invention relates to rimmed steel products and methods of producing them, being products resulting from rolling operations, i.e. hot rolling and for presently preferred uses, also cold rolling with appropriate annealing. The invention is more particularly related to rimmed steel products of the stated character, which are essentially non-aging, the latter term being used herein as equivalent to retarded aging, in the sense of aging being so retarded, e.g. under expected temperatures of use, as to have no significantly adverse effect on the properties of the steel or articles formed from it over indefinite periods of time.
It has long been known that rimmed steel, although having many virtues in the areas of ingot metal yield, superior surface, and good qualities of forming, e.g. drawing, bending and the like, and also articles formed from such steel, are susceptible to so-called aging which affects the mechanical properties in a manner which may be undesirable for some uses. It has also long been known that addition of vanadium, i.e. to the molten steel in the ladle before pouring ingots, has the effect of retarding aging in the ultimate products, resulting in what have been called non-aging rimming steels; in such case, the vanadium has been stated to combine with nitrogen while permitting a mild rimming action. Addition of boron in like manner has been known to have similar effect in retarding aging.
It is found, nevertheless, that the non-aging, vanadium-added or boron-added steels as just described, although purportedly of rimmed character, are deficient especially in that the surface zone is by no means equivalent to that of ordinary or plain carbon, rimmed steel. The clear, clean surface of normal rimmed steel is extremely important, for example in such uses as automotive body stock, where a formed surface free from irregularities is required; the tendency of the previous vanadium-added (or boron-added) steels is to exhibit relatively poor surfaces, unsuitable for the purposes. A special requirement is for a rimmed steel sheet which in cold-rolled, annealed, and temper rolled condition exhibits little or no Luders strain (with characteristic surface lines) and which after forming, and after an interval that normally involves aging, exhibits no appearance of Luders strain, returned. Hence, there has remained a need for an essentially non-aging rimmed steel having the advantages of such steel, in production and use, and having excellent surface characteristics especially in the cold-rolled, annealed and temper rolled condition.